I have two programs:

  • server ... it generates UDP traffic on a chosen multicast
  • listener ... it prints UDP traffic on a chosen multicast (it subscribes to a multicast and prints whatever it receives).

When I run the server on one machine and listeners on some (other) machine(s), the listener sees UDP traffic and prints it correctly. So these programs should be in a good shape.

However, when I try to capture the traffic, on whatever machine, with tcpdump:

sudo tcpdump -i eth0 'dst 233.65.120.153' -w 0.pcap

and when I later try to replay it, on whatever machine, with tcpreplay:

sudo tcpreplay -i eth0 0.pcap

none of the listeners sees those captured packets:

09:38:40.975604 IP (tos 0x0, ttl 1, id 0, offset 0, flags [DF], proto UDP (17), length 32)
    172.27.6.176.53507 > 233.65.120.153.64968: [udp sum ok] UDP, length 4
    0x0000:  4500 0020 0000 4000 0111 6527 ac1b 06b0  E.....@...e'....
    0x0010:  e941 7899 d103 fdc8 000c 579c 6162 6364  .Ax.......W.abcd
    0x0020:  0000 0000 0000 0000 0000 0000 0000       ..............
09:38:41.975709 IP (tos 0x0, ttl 1, id 0, offset 0, flags [DF], proto UDP (17), length 32)
    172.27.6.176.53507 > 233.65.120.153.64968: [udp sum ok] UDP, length 4
    0x0000:  4500 0020 0000 4000 0111 6527 ac1b 06b0  E.....@...e'....
    0x0010:  e941 7899 d103 fdc8 000c 579c 6162 6364  .Ax.......W.abcd
    0x0020:  0000 0000 0000 0000 0000 0000 0000       ..............
09:38:42.975810 IP (tos 0x0, ttl 1, id 0, offset 0, flags [DF], proto UDP (17), length 32)
    172.27.6.176.53507 > 233.65.120.153.64968: [udp sum ok] UDP, length 4
    0x0000:  4500 0020 0000 4000 0111 6527 ac1b 06b0  E.....@...e'....
    0x0010:  e941 7899 d103 fdc8 000c 579c 6162 6364  .Ax.......W.abcd
    0x0020:  0000 0000 0000 0000 0000 0000 0000       ..............

Note that even though none

udp multicast tcpdump
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